The present invention relates to a battery using lithium metal as a negative electrode, specifically to a secondary battery with a superior cycle life in which the growth of a dendrite can be suppressed.
A lithium battery using a lithium metal as a negative electrode has excellent energy density. However, a dendrite grows on the surface of the lithium metal serving as a negative electrode upon charging, and the dendrite may penetrate a separator and cause short circuit between the negative electrode and the positive electrode, resulting in failure of the function of the battery. Additionally, the short circuit may cause an abnormal reaction, and, as a result, causes a problem on the safety of the battery. The growth of the dendrite may deteriorate the cycle characteristics of the battery.
Hence, the method for preventing the dendrite growth has been proposed that a lithium alloy is formed by mixing the lithium metal with other components such as aluminum, bismuth, lead, tin, and indium, or a lithium oxide film is formed on the surface of the lithium metal. According to these methods, however, the operating voltage of the battery is lowered and the energy density is smaller as compared with a negative electrode made of the lithium metal.
Moreover, Japanese Patent Laid-Open No. 7-296812 discloses the proposal, instead of using the lithium metal foil formed by rolling; the amorphous lithium or the amorphous lithium alloy is used for the negative electrode.
The surface of the metallic lithium in an amorphous state of such negative electrode is made to be less likely to form a highly active reaction sites such as a crystal grain boundary, which is a singular point of the growth of dendrite. However, a battery with excellent characteristics cannot be obtained only by making an amorphous state in the negative electrode.
An object of the present invention is to provide lithium metal in a secondary battery, which is stable, is inhibited to grow dendrite shape. Such a secondary battery shows a superior energy density and a superior electromotive force upon long cycling.
Regarding a lithium secondary battery having a lithium metal or a lithium alloy as a material in a negative electrode, the drawback is overcome by the present invention. In the present invention, the battery has the negative electrode comprising of the lithium metal or the lithium alloy formed on the conductive substrate, the lithium metal or the lithium alloy formed by vacuum film-forming, or the amorphous metallic lithium or the amorphous lithium whose surface is coated with a hydrophobic material layer.
Further, according to the above-mentioned lithium secondary battery in the present invention, the hydrophobic material layer comprises at least a material selected from hydrocarbon and ester. The carbon atom in the hydrocarbon or the ester may be partially substituted with silicon atoms, or the hydrogen atoms in the hydrophobic material may be partially or entirely substituted with the fluorine atoms.
Additionally, according to the above-mentioned lithium secondary battery in the present invention, the hydrophobic material layer is formed on 90% or more of the surface of the negative electrode.
According to the above-mentioned lithium secondary battery in the present invention, the ester is made of at least a material selected from fatty ester, phenylcarboxylic acid ester, and diester. The carbon atoms in the ester may be partially substituted with silicon atoms, or the hydrogen atoms in the ester may be partially or entirely substituted with the fluorine atoms.
According to the above-mentioned lithium secondary battery in the present invention, the hydrophobic material is at least a material selected from a fluorine carboxylic acid ester, a phthalate ester, and a benzoic ester. Silicon atoms may partially substitute for the carbon atoms in these esters, or fluorine may substitute for hydrogen partially or entirely in these esters.
According to the above-mentioned lithium secondary battery in the present invention, the hydrophobic material is a material such as dioctyl phthalate, cetylnaphthalene, carboxylic acid ester, fluorine carboxylic acid ester, or neroli oil.
Further, according to the manufacturing method of the lithium secondary battery using lithium metal or a lithium alloy as a negative electrode in the present invention, the negative electrode is opposed to a positive electrode via a separator. The negative electrode is formed by the deposition of the lithium metal (or its alloy) or the amorphous metallic lithium (or its alloy) on the conductive substrate by vacuum film-forming. The negative electrode may be coated with the hydrophobic material by a method of dipping in a solution containing the hydrophobic material, by the sputtering method or by the vapor deposition method.